Most lighting products in both the professional and private market consist of fluorescent lamps. Mostly, these fluorescent lamps are fluorescent tubes. A fluorescent lamp utilizes a fluorescent substance which converts ultraviolet radiation in the lamp into visible light. The efficiency of the fluorescent lamps is higher than that of the incandescent lamps, i.e. 90 Lumen/Watt versus 12 Lumen/Watt. Despite all this, since the introduction of the LED lamps, the fluorescent lamps are no longer the most energy efficient lighting technique. Moreover, the operational lifetime of the fluorescent lamps is relatively short, i.e. about 5000 operation hours, which implies regular replacement.
Every year 12.5 million fluorescent lamps are released on the Dutch market. In practice, replacement schemes for fluorescent lamps are such that they are replaced on a fixed moments in time before their technical life span is over. This happens, amongst other places, in companies and hospitals.
In fluorescent tubes the fluorescent coating (mostly phosphor) is provided on the inside of a glass tube, which is filled with an inert gas, such as Argon, Krypton, or a mixture of both, and mercury vapor (which is very poisonous). The mercury vapor emits ultraviolet light in case of a gas-discharge (which is initiated by putting a voltage over electrodes on two opposing end of the tube). The ultraviolet light is subsequently converted into visible light by the fluorescent coating on the inside of the tube.
It is not possible to connect a fluorescent tube directly to the lighting mains. This requires a starter and a choke (VSA), which are to be included in the armature. More recent fluorescent tubes are driven by an electronic choke (which uses very high switching frequencies). In any case such choke is also to be included in the armature.
LED technology, a form of sold-state lighting (SSL), is developing fast, especially in the application areas of signal lighting (traffic lights, monitoring lights, etc) and for small scale compact light sources (for example reading lamps, flash lights, or decoration lights). The development of LED towards full-fledged light source has not yet been possible due to technological barriers, such as the incompatibility with existing lighting installations/armatures.
Market leaders on the light market focus on the development of LED lighting in new systems, while focusing on traditional lighting technology (incandescent lamps, fluorescent lamps, low-energy bulbs) in the after market. They do not seem to realize that LED lighting can be a sustainable alternative for existing light sources in this market as well.
When switching from fluorescent lighting technology towards another technology (such as LED technology) compatibility plays a major role. There is a major reluctance to step over to a new lighting technology due to large investments which have been done in armatures and installations, despite possible economic and ecologic advantages. Expressed differently, market acceptance for a new lighting technology is only quickly obtained on a short term where the lighting technology is fully replaceable with the existing lighting technology. Even the looks of the new lighting products must resemble the old ones. Thus there is a need for an environment-friendly lighting technology which can be replaced one-on-one with the current fluorescent tube systems.
The current compatibility problems with the retrofit-range are caused by the following aspects:                there is limited physical space for components, i.e. new lighting products need to be implemented in existing systems and therefore need to have the same dimensions as the current lighting products;        the shape of the new products must resemble the shape of the old products. For the end-user the shape of the new product must be recognizable before he is willing to accept the new product. This aspect puts severe design constraints in LED lighting products.        
Because of the above-mentioned additional requirements to retrofit-systems, one of the most important aspects of an LED lamp which is at stake is the cooling of the LED's. LED's produce a significant amount of heat. Cooling systems are required for keeping the temperature of the LED's within acceptable limits. Active cooling systems, which have been reported in the prior art, are not desired in lighting applications because of several disadvantages which it has as well (more components which may get broken, i.e. reduced operational lifetime, noise, not enough space, etc).
Different so-called retrofit LED tubes have been disclosed in the prior art. Most of the known LED tubes comprise a plurality of low-power LED's connected in series, i.e. several hundreds of LED's. These LED tube can be mounted into a conventional lighting fixture having fluorescent tubes connected in parallel (using the full mains voltage).
A problem with the known LED tube is that they are not fully retrofit. The known LED tube is not suitable for being used in lighting fixtures having a plurality of LED tubes in series, and in particular two LED tubes.